if it wasn't meant for the public, they shouldn't have uploaded their image to the internet in the first place besides that, as long as you don't copy a whole image, everything's fine.if you are observing an image to get your pose or facial expression or whatever right, you don't have to worry about an image being stock or not, since you are just watching at the image and figuring out how you can draw something correctly, you're not stealing anything at all.

Excuse me while I go kick the CEO of Viacom in the junk for forcing DirecTV to remove Nick from the lineup so no LoK (Comedy Central too, so no Futurama either! D: ) hopefully they get it resolved before season 2 of LoK starts.